Optical Disc Apparatus

ABSTRACT

In servo control within an optical disc apparatus, when iterative learning control is started, the servo characteristics are modified to servo characteristic giving increased gains during a given period of time. Under this condition, the iterative learning control is provided.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2007-321487 filed on Dec. 13, 2007, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disc apparatus for recordingand reading information to and from an optical disc.

Periodic external disturbance caused by rotation of an optical disc isone factor causing a deviation in servo tracking performed in an opticaldisc apparatus. A known method of improving the capability to followsuch periodic disturbance is iterative learning control (see, forexample, JP-A-9-50303).

However, according to JP-A-9-50303, in the iterative learning controlsystem, a control output is created for compressingrotation-synchronized vibrations using a position error signal producedone rotation earlier. It is known that during one rotation while theiterative learning control is in operation, the iterative learningcontrol produces no effect.

Therefore, there is the problem that the repetitive compensator producesno action until a control error signal corresponding to one rotation ofdisc is accumulated in a memory inside the repetitive compensatorwhenever the mode of operation is switched to tracking control modeimmediately after a seek operation. In an attempt to solve this problem,JP-A-2003-249045 discloses a method in which a repetitive compensatorcontinues learning using values learned during tracking control prior tostart of a seek operation as initial values immediately after a trackingoperation is resumed subsequent to completion of the seek operation.

It is known that periodic external disturbances produced when an opticaldisc rotates include a component known as a deviation, in addition toeccentric and surface wobble components. JP-A-2007-207390 sets forth,“An optical disc having guide grooves may sometimes have ill-shapedgroove portions (deviations) due to deterioration of the disc stamper orinferior formation of the disc. Where such a disc is rotated at a highmultiple speed, wide-band peculiar noise components are introduced intothe groove-reflected signal, especially near the outer periphery.”

SUMMARY OF THE INVENTION

In a servo error signal used for control of a focus and tracking system,components not fully suppressed by the servo system appear as a residualerror. Therefore, where the optical disc has surface wobble andeccentricity, the servo error signal is observed to contain a varyingsignal component of the rotational frequency. Similarly, where sometracks of the optical disc have distortion (deviations), the servo errorsignal is observed as a high-frequency, varying signal component.

FIG. 10 is a schematic diagram of a servo error signal produced when thebeam passes across a deviated region. A high-frequency varying signalcomponent that is observed in a servo error signal when the beam passesacross such a deviated region is hereinafter referred to as a deviationcomponent. As shown in FIG. 10, the servo error signal containseccentric and surface wobble components varying at the rotationalfrequency, as well as the deviation component indicated by A. Thedeviation component is produced in synchronism with the period ofrotation and has the feature that a substantially equal variable signalwaveform appears from period to period of rotation, for the followingreasons. The deviation is a local distortion arising from themanufacturing process of the optical disc, and is produced at a certainangle during one rotation of the optical disc. Another reason is thatadjacent tracks are distorted into substantially identical forms.Because the deviation is a local distortion, even at the same angle onan optical disc, the deviation component is present and observed at afirst radial position but not observed at a second radial positiondefining a different radius.

Since such a deviation component is a signal waveform synchronized withthe period of rotation, the deviation component can be represented as asum of a rotational frequency component and components of its harmonicsby Fourier transform. Consequently, the deviation component can besuppressed by iterative learning control. However, as mentionedpreviously, the effects of the repetition control do not appear at leastduring one rotation from the start of the operation. The suppressiondepends only on the feedback control means. Hence, there is the problemthat the degree of suppression achieved by the servo system is lowerthan in the steady state where the effects of the iterative learningcontrol have appeared.

The physical accuracies in the focus direction and tracking direction oftracks traced by an optical disc apparatus are stipulated as opticaldisc standards. In practice, however, there is the possibility thatoptical discs having deviations not satisfying the standards are presenton the market. In order to cope with such optical discs, the opticaldisc apparatus is required to have a servo system having better trackingperformance than assumed by the standards. Meanwhile, there is a demandfor a technique of shortening recording and reading times by using ahigher multiple speed than required by the standards for optical discscomplying with given standards. Also, in this case, better trackingperformance than assumed by the standards is required.

Where tracking performance better than assumed by the standards isrequired as the performance of the servo system of an optical discapparatus, the inventors of the present application found that thesuppression of the gain becomes especially insufficient for externaldisturbances of high frequencies such as deviation components. Theinventors of the present application have found that in the worst case,it is impossible to follow the tracks and the servo control becomesineffective.

Where a seek operation is performed on a region where a high-frequencydeviation component is present, for example, the optical beam passesacross the deviation portion during the period of one rotation until theeffects of iterative learning control appear. There is the danger thatthe servo control becomes ineffective. Even if the servo control doesnot become ineffective, surface wobble and eccentric components anddeviation components are less suppressed under the condition where theiterative learning control is not yet effective. There is thepossibility that the reading performance deteriorates and that addressinformation cannot be read from the optical disc after the seek and thusthe seek operation ends unsuccessfully. The degree of suppressionachieved by the servo control is insufficient until the period of onerotation elapses after an iterative learning control operation isstarted. This may lead to a severe situation where the servo controlbecomes ineffective in cases where iterative learning control is startedat a radial position where a deviation component is present.

The technique disclosed in JP-A-2003-249045 is available as a method ofimproving the tracking performance immediately after iterative learningcontrol is started. This technique is based on the premise that therotation-synchronized component stored in a memory during rotationsprior to a seek operation does not vary after the seek operation. In thecase of deviations, they are local distortions of tracks in the opticaldisc and so the varying component of the error signal occurring duringthe rotation at a radial position immediately after a seek operation isdifferent from the varying component occurring during the rotation at adifferent radial position prior to the seek operation. Consequently, thetechnique disclosed in JP-A-2003-249045 produces insufficientsuppressing effects.

In view of the foregoing problem, it is an object of the presentinvention to provide an optical disc apparatus which uses iterativelearning control and which exhibits improved tracking performance.

The object of the present invention can be achieved, for example, byincreasing the servo gain of a feedback control module for a givenperiod of time after iterative learning control is started.

According to the present invention, the tracking performance of anoptical disc apparatus can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram showing Embodiment 1 of the present invention;

FIG. 2 is a diagram of a storage circuit of Embodiment 1 of theinvention;

FIG. 3 is a block diagram of a focus control circuit of Embodiment 1 ofthe invention;

FIG. 4 is a flowchart illustrating a sequence of operations performed inEmbodiment 1 of the invention;

FIG. 5 is a waveform diagram illustrating a servo gain used in amodification of Embodiment 1 of the invention;

FIG. 6 is a waveform diagram illustrating Embodiment 1 of the invention;

FIG. 7 is a flowchart illustrating a sequence of operations performed inEmbodiment 2 of the invention;

FIG. 8 is a waveform diagram illustrating Embodiment 2 of the invention;

FIG. 9 is a waveform diagram illustrating a servo gain used in amodification of Embodiment 2 of the invention; and

FIG. 10 is a waveform diagram illustrating a deviation component of aservo error signal.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the optical disc apparatus of the present invention arehereinafter described with reference to the drawings.

The present invention can be applied to both tracking control and focuscontrol. In the following description, focus control is taken as anexample.

Embodiment 1

Embodiment 1 of the present invention is described below. Embodiment 1is a mode of practice for solving a first problem.

FIG. 1 is a block diagram showing main portions of an optical discapparatus of the present embodiment.

Indicated by reference numeral 101 is an optical disc from whichinformation is read by laser irradiation. Also, information held on thedisc is erased by laser irradiation. Furthermore, information can bewritten onto the disc by laser irradiation.

Indicated by reference numeral 102 is an objective lens that focuseslaser light onto the recording surface of the optical disc 101 byfocusing the laser light.

Indicated by reference numeral 103 is an optical pickup equipped with afocus actuator (not shown). The pickup is also equipped with an opticaldetector (not shown) that detects light reflected from the optical disc101 and outputs an electrical signal corresponding to the amount of thereflected light.

Indicated by reference numeral 104 is a spindle motor that rotationallydrives the optical disc 101 at a given linear velocity. The period ofrotation is hereinafter indicated by T_(rot).

Indicated by reference numeral 105 is a focus error signal creationcircuit that creates a focus error signal V101 from the output signalfrom an optical detector incorporated in the optical pickup 103 andoutputs the signal.

Indicated by reference numeral 106 is an adder that adds the focus errorsignal V101 and an output signal from a storage circuit 110 (describedlater).

Indicated by reference numeral 107 is a focus control circuit thatcompensates the output signal from the adder 106 for gain and phase andcreates a drive signal V102. Furthermore, the focus control circuit 107modifies filter characteristics according to an instruction from aniterative learning control circuit 111 (described later).

Indicated by reference numeral 108 is a driver circuit that amplifiesthe drive signal V102 from the focus control circuit 107 and suppliesthe amplified signal to the focus actuator incorporated in the opticalpickup 103.

Indicated by reference numeral 109 is a rotation-synchronized signalcreation circuit, which creates a rotation-synchronized signal V103having a duty cycle of 50% and a period of T_(rot)/N (where N is anatural number) from the output signal from the spindle motor 104.

Indicated by reference numeral 110 is the storage circuit, which acceptsthe output signal from the adder 106 based on the rotation-synchronizedsignal V103, divides one rotation by N, and stores the value of thesignal. Furthermore, the storage circuit outputs a compensation signalV104 for compensating for periodic external disturbances entered to theservo system based on the stored value. In addition, the storage circuitdiscards the stored values, based on a memory resetting signal V105outputted from the iterative learning control circuit 111 (describedlater).

Indicated by numeral 111 is the iterative learning control circuit thatcontrols iterative learning control. The iterative learning controlcircuit 111 gives an instruction to discard the value stored in thestorage circuit 110 by the use of the memory resetting signal V105. Theiterative learning control circuit 111 has an internal counter having afunction of measuring the elapsed time from execution of a desiredoperation. Furthermore, the iterative learning control circuit 111calculates the present period of rotation using therotation-synchronized signal V103 as its input. For example, this can becalculated by measuring the elapsed time until the rising edge of therotation-synchronized signal V103 is counted N times. As an example, ageneral CPU can be used as the iterative learning control circuit 111.

The configuration of the storage circuit 110 of the present embodimentis described by referring to FIG. 2. The storage circuit 110 is sodesigned that it is composed of N memory devices 201 connected inseries. For example, each memory device 201 can be made of a Dflip-flop. The storage circuit 110 outputs the value stored in the Nthmemory device.

When the rising edge of the rotation-synchronized signal V103 isdetected, each memory device 201 performs a shift register operation,i.e., the value stored in the previous stage of memory device is shiftedto the following stage of memory device. In this way, the N memorydevices 201 are successively updated at time intervals of T_(rot)/N.When the rising edge of the memory resetting signal V105 is detected,the N memory devices 201 constituting the storage circuit 110 discardthe values held in the respective memory devices and overwrite with azero level. This operation consisting of overwriting each memory devicewith a zero level is hereinafter referred to as resetting of the memorydevice.

It is assumed that in the present embodiment, the total number N of thememory devices constituting the storage circuit 110 is large enough tostore the profile of the deviation component contained in the focuserror signal V101.

The configuration of the focus control circuit 107 of the presentembodiment is next described by referring to FIG. 3. In the focuscontrol circuit, a gain control device 301 providing a variable gain(also indicated by 301), a phase advance compensator 302, and a phaselag compensator 303 are connected in series.

A value set by the gain control device 301 under steady state asdescribed later is herein referred to as a steady-state gain value. Thegain control device 301 modifies the value of the gain underinstructions from the iterative learning control circuit 111.

In the following, an operation for storing the value of a signal intothe storage circuit 110 is defined to be included in “iterative learningcontrol”. According to this definition, “start of iterative learningcontrol” is the “timing at which the output signal from the adder 106 isstarted to be recorded into the storage circuit 110”. Storage of asignal corresponding to one rotation of the optical disc into thestorage circuit 110 is not completed until at least one period ofrotation elapses after the “start of iterative learning control”. Underthis state, the effects of the iterative learning control begin toappear in which the value of the signal stored in the immediatelyprevious rotation was returned to the original value.

The operation of the iterative learning control circuit 111 at the startof iterative learning control according to the present embodiment isnext described by referring to the flowchart of FIG. 4.

When the iterative learning control is started (step S401), theiterative learning control circuit 111 increments the value of thevariable gain 301 (step S402).

Then, the values of all the memory devices of the storage circuit 110are reset (step S403). Thereafter, the value obtained after resettingthe memory devices is read out as the compensation signal V104 until theperiod of one rotation elapses and so a zero level is outputted.Meanwhile, after execution of step S403, the value of signal is startedto be recorded in the storage circuit 110. Therefore, if an intervallonger than the period of one rotation elapses after step S403, thevalue of signal is recorded up to the Nth memory device of the storagecircuit 110. As a result, a signal created based on the value of thesignal stored during one previous rotation is started to be outputted asthe compensation signal V104. That is, in the present embodiment, stepS403 corresponds to start of iterative learning control.

Then, a decision is made as to whether the period of one rotation haselapsed after resetting the values of the memory devices (step S404). Ifthe decision at step S404 is negative (NO), control returns to stepS404. On the other hand, if the decision at step S404 is affirmative(YES), the value of the variable gain 301 is returned to thesteady-state gain value (step S405), thus completing the sequence ofoperations to be conducted at the start of iterative learning control(step S406).

The advantages of the present embodiment are described by referring toFIG. 5, which is a schematic diagram illustrating the case whereiterative learning control is started at a radial position of an opticaldisc where a deviation is present. The time axis is described, assumingthat the timing (corresponding to step S403) at which the memory devices201 are reset is t=0.

In the present embodiment, the iterative learning control does not workuntil the period of one rotation elapses after iterative learningcontrol is started and a signal is started to be stored in the storagecircuit. During this period (0≦t≦T_(rot)), the value of the variablegain 301 increases (see (a) of FIG. 5). This in turn increases the servogain of the feedback control system. Consequently, both surface wobbleand deviation components are more suppressed.

In (b) of FIG. 5, the broken line indicated by A shows variation of thefocus error signal V101 occurring when the value of the variable gain301 is not increased. Note that the shown variation is only for theinterval, 0≦t≦T_(rot). In contrast, in the present embodiment, the valueof the variable gain 301 is increased during the interval, 0≦t≦T_(rot),and so the variation in the focus error signal V101 occurring when thebeam passes through the deviation portion during the interval,0≦t≦T_(rot), is suppressed (see ( ) of FIG. 5).

On the other hand, during the interval, T_(rot)≦t, the value of thevariable gain 301 returns to its steady-state gain value. However, thecompensation signal V104 based on the value of the signal stored duringone previous rotation is started to be outputted. Therefore, during theinterval, T_(rot)≦t, the effects of iterative learning control begin toappear. The range of variation of the error signal in the deviationportion is suppressed.

As a result, even where a deviation is present during the period betweenthe start of iterative learning control and the end of one rotation, thetracking performance can be improved.

In the description provided so far, the operation is performed toincrease the variable gain 301 within the focus control circuit 107. Thefilter characteristics of the phase advance compensator 302 and phasedelay compensator 303 may also be varied together with the variable gain301. In this case, the filter characteristics may be so varied that theopen loop servo characteristics give increased gains at low frequenciesas shown in the diagram of the servo gain of FIG. 6.

In FIG. 6, f_(rot) indicates the rotational frequency. A frequency rangecontaining the deviation component is given by f₁<f<f₂ (where f is afrequency). In the case of open-loop servo characteristics givingincreased gains at low frequencies as shown in FIG. 6, surface wobblecomponent and deviation component are more suppressed than steady-stateopen loop servo characteristics. Consequently, the same advantages asthe foregoing advantages are obtained. The servo characteristics givingincreased gains at low frequencies as shown in FIG. 6 can beaccomplished by modifying the filter characteristics of the phase delaycompensator 303 to characteristics giving increased gains at lowfrequencies.

The filter characteristics are so determined that the gain is increasedat low frequencies as shown in FIG. 6. Consequently, increases in servogain are limited to low-frequency components compared with the casewhere the gain is increased using only the gain control device 301. As aresult, two advantages are obtained. A first advantage is that the servocontrol can be stabilized at the sub-resonance frequency because theservo gain does not increase at the sub-resonance frequency of theactuator lying in a high-frequency range. Another advantage is that thecurrent consumption of the optical disc apparatus can be suppressedbecause undesired amplification of high-frequency components by thefocus control circuit 107 is suppressed.

The focus control circuit 107 switches the used filter circuit betweenplural filter circuits. This eliminates the time taken to modify thefilter characteristics. Consequently, it is assured that the servocontrol is stabilized when the filter characteristics are modified.

In the description provided so far, the variable gain 301 is increasedduring the period of one rotation after resetting of the values of thememory devices. The variable gain 301 is returned to its steady-stategain value simultaneously with appearance of the effects of iterativelearning control. The interval during which the variable gain 301 isincreased is not limited to the period of one rotation. The interval mayalso be the period of more than one rotation. That is, the operation maybe so performed that the variable gain 301 is returned to itssteady-state gain value after the instant (t=T_(rot)) at which theeffects of iterative learning control begin to appear.

Because of the operations described so far, the optical disc apparatusaccording to Embodiment 1 compensates for lack of trackability inducedimmediately after the start of iterative learning control by increasesin variable gain 301. As a result, the tracking performance of the servosystem can be enhanced.

Embodiment 2

Embodiment 2 of the present invention is described below.

In Embodiment 1, the operation is so performed that the gain is returnedto the steady-state gain value after a lapse of a given time since thevariable gain 301 was increased. However, there is a difference in openloop gain characteristic of servo system between the first servocharacteristic in which the variable gain 301 is increased and thesecond servo characteristic in which the variable gain 301 is returnedto the steady-state gain value and iterative learning control is used.Accordingly, the waveform of variation of the error signal produced whenthe same track displacement is entered is different according to whichservo characteristic is used.

Therefore, in the already described Embodiment 1, the variable gain 301is returned to the steady-state gain value. The compensation signal V104created based on the waveform indicating variation of the error signalproduced when the beam passed with the first servo characteristic onerotation earlier (0≦t≦T_(rot) in FIG. 5) is applied to the servo loopuntil the optical disc rotates once (T_(rot)≦t<2T_(rot) in FIG. 5) sincethe servo characteristic was modified to the second servocharacteristic. Under this condition, iterative learning control isprovided. As a result, especially during the rotation(T_(rot)≦t<2T_(rot) in FIG. 5) occurring immediately after the variablegain 301 was returned to the steady-state gain value, the output of theiterative learning control does not suppress variation of the errorsignal. Rather, there is the possibility that the suppressingperformance is deteriorated.

However, whether the suppressing performance deteriorates in Embodiment1 depends on the amount of increase in the variable gain 301, also onthe suppressing performance of iterative learning control, andfurthermore on the frequency of the deviation component actually presenton the optical disc 101 and the profile of the waveform indicatingvariation of the error signal when the beam passes through the deviationregion. Therefore, the suppressing performance is not alwaysdeteriorated even in the optical disc apparatus shown in Embodiment 1.

In the present embodiment, the stability of the servo control isimproved immediately after start of iterative learning control in viewof the foregoing problems.

A block diagram showing an optical disc apparatus of the presentembodiment is common with FIG. 1 that is a block diagram of an opticaldisc apparatus of Embodiment 1.

The operation of the iterative learning control circuit 111 performedwhen iterative learning control of the present embodiment is started isdescribed below by referring to the flowchart of FIG. 7.

When iterative learning control is started (step S701), the iterativelearning control circuit 111 increases the value of the variable gain301 (step S702).

Then, the values of all the memory devices of the storage circuit 110are reset (step S703). Thereafter, the values obtained after resettingthe memory values are outputted as the compensation signal V104 untilthe period of one rotation elapses and so a zero level is outputted. Onthe other hand, after step S703, recording of the value of the signalinto the storage circuit 110 is started. Therefore, when an intervallonger than the period of one rotation elapses after step S703, thevalue of the signal is recorded up to the Nth memory device of thestorage circuit 110. As a result, a signal based on the value of thesignal stored during one previous rotation is outputted as thecompensation signal V104. That is, in the present embodiment, step S703corresponds to start of iterative learning control.

A decision is made as to whether the period of M rotations (where M is anatural number) has passed since the values of the memory devices werereset (step S704). If the decision at step S704 is NO (i.e., the periodof M rotations has not passed since the values of the memory deviceswere reset), the value of the variable gain 301 is modified to a valuecorresponding to the elapsed time from the resetting of the memorydevices (step S705). At this time, the value of the variable gain 301decreases with the elapsed time. A method of modifying the variable gain301 in the present embodiment will be described later.

If the period of M rotations has elapsed since the values of the memorydevices were reset (decision at step S704 is YES), the value of thevariable gain 301 is returned to the steady-state gain value (stepS706), thus completing the sequence of operations to be carried out atthe start of iterative learning control (step S707).

The advantages of the present embodiment are described by referring toFIG. 8, which is a waveform diagram illustrating the case whereiterative learning control is started at a radial position of an opticaldisc where a deviation is present. With respect to the time axis, it isassumed that the instant (corresponding to step S703) at which thevalues of the memory devices 201 are reset is set to t=0.

FIG. 8 shows a case where M=3. The variable gain 301 is modified in stepS705 of the present embodiment at the instant when the elapsed time fromresetting of the memory devices 201 becomes equal to an integralmultiple of the period of rotation. The modified value is reducedlinearly using a logarithmic scale.

Specifically, let G_(norm) be the steady-state gain value. Let G₁ be thevalue of the variable gain 301 modified in step S702. Let t (t<4T_(rot))be the elapsed time from resetting of the memory devices 201. The valueG_(val) of the variable gain 301 is determined according to thefollowing Eq. (1):

$\begin{matrix}{{G_{val}(t)} = {G_{norm} \times \left( \frac{G_{1}}{G_{norm}} \right)^{1 - \frac{{int}{({t/T_{rot}})}}{3}}}} & (1)\end{matrix}$

where int (N) is a function returning a maximum integer not exceeding N.As an example, it is here assumed that the value G₁ is greater thanG_(norm) by 3 dB. At this time, G_(val) is larger than G_(norm) by 3 dBduring the interval, 0≦t<T_(rot). G_(val) is larger than G_(norm) by 2dB during the interval, T_(rot)≦t<2T_(rot). G_(val) is larger thanG_(norm) by 1 dB during the interval, 2T_(rot)≦t<3T_(rot). By reducingthe variable gain 301 in a stepwise fashion as shown in (a) of FIG. 8,the amount of variation in the servo gain occurring before and after thevariable gain 301 was modified can be set to a small value of 1 dB.

In (b) of FIG. 8, the broken line A indicates variation of the focuserror signal V101 when the value of the variable gain 301 is notincreased. Note that the shown variation is only for the interval,0≦t<T_(rot). In contrast, in the present embodiment, the value of thevariable gain 301 is increased during the interval, 0≦t<T_(rot), and sothe range of variation in the focus error signal V101 occurring when thebeam passes through the deviation portion during the interval,0≦t≦T_(rot), is suppressed.

In the present embodiment, when the value of the variable gain 301 ismodified at t=T_(rot), the amount of variation is reduced. As a result,the difference in servo characteristics between the first rotation(0≦t<T_(rot)) after resetting of the memory devices and the secondrotation (T_(rot)≦t<2T_(rot)) decreases. For this reason, the waveformindicating variation of the focus error signal V101 on passing throughthe deviation region becomes similar from period to period. As a result,the suppressing effects of iterative learning control on variation ofthe focus error signal V101 improves when the beam passes through thedeviation region during the second rotation.

Similarly, the difference in servo characteristics between the secondand third rotations is reduced. The difference in servo characteristicsbetween the third and fourth or following rotations is reduced.Consequently, the output of the iterative learning control can suppressvariations in the focus error signal V101.

The variable gain 301 is reduced with a lapse of time in this way. Thisis equivalent to gradually approaching the servo characteristics to thesteady-state servo characteristics. As a result, the output of iterativelearning control gradually varies with each rotation. Outputting of theiterative learning control can be started while suppressing variationsin the focus error signal V101 on passing through the deviation regionduring each rotation.

In the operation described so far, the gain is reduced over the periodof 3 rotations by assuming that M=3. The interval is not limited to theperiod of 3 rotations.

In the operation described so far, the variable gain 301 is reducedaccording to Eq. (1) above. The method of modifying the value of thevariable gain 301 is not limited to Eq. (1). In the above operation, thevariable gain 301 is modified with each period of rotation. The variablegain 301 may also be modified every integral multiple of the period of arotation (e.g., every 2 rotations). In addition, the intervals at whichthe value of the variable gain 301 is modified are not limited to anintegral multiple of the period of a rotation.

Furthermore, in the above operation, the variable gain 301 is reducedstepwise after the elapsed time from resetting of the memory devices 201becomes an integral multiple of the period of a rotation. Alternatively,the value of the variable gain 301 may be reduced continuously based onthe elapsed time.

Additionally, in the operation described so far, the variable gain 301within the focus control circuit 107 is modified. The filtercharacteristics of the phase advance compensator 302 and phase delaycompensator 303 may be modified, in addition to the value of thevariable gain 301. In this case, the filter characteristics may bemodified to the filter characteristics G₁(s) in which the open loopservo characteristics give increased gains at low frequencies as shownin the servo gain diagram of FIG. 9. Plural filter characteristicsG₂(s), . . . , G_(M)(s) in which the amount of increase in the gain isreduced at low frequencies may be prepared. In step S705, the filtercharacteristic may be modified successively to filter characteristicsproviding successively reduced amounts of gain increase with a lapse oftime. In FIG. 9, f_(rot) indicates the rotational frequency. Thefrequency range in which a deviation component is contained is set tof₁<f<f₂ (where f is a frequency). As shown in FIG. 9, servocharacteristics providing increased gain values only at low frequenciescan be accomplished, for example, by modifying the filter characteristicof the phase delay compensator 303 to a characteristic providingincreased gains at low frequencies.

Increases in servo gain are limited to low-frequency components unlikethe case where the gain is increased using only the gain control device301, by setting the filter characteristic in such a way that the gain isincreased at low frequencies as shown in FIG. 9. As a result, twoadvantages are obtained. A first advantage is that the servo control canbe stabilized at the sub-resonance frequency because the servo gain doesnot increase at the sub-resonance frequency of the actuator lying in ahigh-frequency range. Another advantage is that the current consumptionof the optical disc apparatus can be suppressed because undesiredamplification of high-frequency components by the focus control circuit107 is suppressed.

Because of the operations described so far, the optical disc apparatusaccording to Embodiment 2 compensates for lack of trackabilityimmediately after iterative learning control by increases in variablegain 301. The tracking performance of the servo system obtainedimmediately after the start of outputting of iterative learning controlcan be improved by gradually returning the variable gain 301 to itssteady-state gain value.

In the embodiment described so far, the focus control circuit 107 isdesigned to include the gain control device 301, phase advancecompensator 302, and phase delay compensator 303 connected in series.The configuration of the focus control circuit 107 is not limited tothis configuration. For example, the phase advance compensator and thephase delay compensator may be connected in parallel behind the gaincontrol device.

In the embodiment described so far, the value of the variable gain 301is modified simultaneously with start of iterative learning control. Thevalue of the variable gain 301 may also be modified before the start ofiterative learning control.

In the embodiments described so far, the period of rotation T_(rot) iscalculated from the output signal from the spindle motor 104. Means ormodule for calculating the period of rotation is not limited to this.For example, address information may be obtained from the output signalfrom the optical pickup 103, and the period of rotation T_(rot) may becalculated from the address information.

In the embodiments described so far, the focus error signal V101 isstored in the storage circuit 110 for an interval corresponding to onerotation of the optical disc. The signal may also be stored for aninterval corresponding to plural rotations.

In the iterative learning control used in the embodiments described sofar, the focus error signal V101 is stored. The drive signal V102 mayalso be stored. Furthermore, signals inside the focus control circuitmay also be stored.

It is obvious that the present invention can also be applied to trackingcontrol.

While there have shown and described several embodiments in accordancewith our invention, it should be understood that disclosed embodimentsare susceptible of changes and modifications without departing from thescope of the invention. Therefor, it is not intended to be bound by thedetails shown and described herein but intend to cover all such changesand modifications that fall within the ambit of the appended claims.

1. An optical disc apparatus for recording or reading information to orfrom an optical disc by irradiating the disc with laser light, saidoptical disc apparatus comprising: an optical detection module toproduce an electrical output signal corresponding to the amount of lightreflected from the optical disc; a servo error signal creation module tocreate a servo error signal from the output signal from the opticaldetection module; a feedback control module to create a drive signal fordriving a servo actuator based on the servo error signal and to providefeedback control; a disc rotation module to rotate the optical disc; anaddition module to add a compensation signal to a given signal within aservo loop, said compensating signal acting to compensate for periodicalexternal disturbances entered to a servo system of the optical discapparatus; a storage module to store an output signal from the additionmodule for an interval of time corresponding to at least one rotation ofthe optical disc; and an iterative learning control module to providecontrol of iterative learning control; wherein said iterative learningcontrol module modifies filter characteristics set to said feedbackcontrol module to given filter characteristics providing increasedgains; wherein outputting of the iterative learning control is startedafter a lapse of the period of at least one rotation after themodification of the filter characteristics; and wherein the filtercharacteristics set to said feedback control module are modified tosteady-state servo characteristics after a lapse of a given time sincethe start of the outputting of the iterative learning control.
 2. Anoptical disc apparatus as set forth in claim 1, wherein said given timeis zero.
 3. An optical disc apparatus as set forth in claim 1, whereinsaid given filter characteristics are servo characteristics giving aservo gain that is increased at least at low frequencies compared withthe steady-state servo characteristics.
 4. An optical disc apparatus asset forth in claim 1, wherein said feedback control module has avariable gain module which amplifies or attenuates the amplitude of aninput signal regardless of the frequency of the input signal and whichoutputs the amplified or attenuated signal, and wherein said givenfilter characteristics are increased values of said variable gainmodule.
 5. An optical disc apparatus as set forth in claim 1, whereinsaid feedback control module has a phase advance compensation module anda phase delay compensation module, and wherein said given filtercharacteristics have been obtained by modifying at least the filtercharacteristics of the phase delay compensation module.
 6. An opticaldisc apparatus for recording or reading information to or from anoptical disc by irradiating the disc with laser light, said optical discapparatus comprising: an optical detection module to produce anelectrical output signal corresponding to the amount of light reflectedfrom the optical disc; a servo error signal creation module to create aservo error signal from the output signal from the optical detectionmodule; a feedback control module to create a drive signal for driving aservo actuator based on the servo error signal and to provide feedbackcontrol; a disc rotation module to rotate the optical disc; an additionmodule to add a compensation signal to a given signal within a servoloop, said compensation signal acting to compensate for periodicalexternal disturbances entered to a servo system of the optical discapparatus; a storage module to store an output signal from the additionmodule for an interval of time corresponding to at least one rotation ofthe optical disc; and an iterative learning control module to providecontrol of iterative learning control; wherein said iterative learningcontrol module modifies filter characteristics set to said feedbackcontrol module to given filter characteristics providing increasedgains; wherein outputting of the iterative learning control is startedafter a lapse of the period of at least one rotation after themodification of the filter characteristics; and wherein the filtercharacteristics set to said feedback control module are modified to onefilter characteristic or plural filter characteristics in turn with thelapse of time after the start of the outputting of the iterativelearning control and then to the steady-state servo characteristics. 7.An optical disc apparatus as set forth in claim 6, wherein said givenfilter characteristics are servo characteristics giving a servo gainthat is increased at least at low frequencies compared with thesteady-state servo characteristics.
 8. An optical disc apparatus as setforth in claim 6, wherein said feedback control module has a variablegain module which amplifies or attenuates the amplitude of an inputsignal regardless of the frequency of the input signal and which outputsthe amplified or attenuated signal, and wherein said given filtercharacteristics are increased values of said variable gain module.
 9. Anoptical disc apparatus as set forth in claim 6, wherein said feedbackcontrol module has a phase advance compensation module and a phase delaycompensation module, and wherein said given filter characteristics havebeen obtained by modifying at least the filter characteristics of thephase delay compensation module.
 10. An optical disc apparatus forrecording or reading information to or from an optical disc byirradiating the disc with laser light, said optical disc apparatuscomprising: an optical detection module to detect the amount of lightreflected from the optical disc; a servo error signal creation module tocreate a servo error signal from an output signal from the opticaldetection module; a feedback control module to create a drive signal fordriving a servo actuator based on the servo error signal and to providefeedback control; a disc rotation module to rotate the optical disc; anaddition module to add a compensation signal to a given signal within aservo loop, said compensating signal acting to compensate for externaldisturbances entered to a servo system of the optical disc apparatus; astorage module to store an output signal from the addition module for aninterval of time corresponding to at least one rotation of the opticaldisc; and an iterative learning control module to provide control ofiterative learning control; wherein said iterative learning controlmodule modifies filter characteristics set to said feedback controlmodule to given filter characteristics providing increased gains; andwherein the filter characteristics set to the feedback control module ismodified to steady-state servo characteristics after a given time andthe iterative learning control is outputted.